1. Field of the Invention
The present invention relates to a method for picture data reduction for digital video signals comprising a preprocessing of the signals by means of block-by-block transformation whereby a transformed and quantatized signal which was generated at a time t-1 and deposited in an image storage is subtracted from a transformed signal which occurs at a time t and whereby the difference signal obtained in such manner is subjected to a quantization.
2. Description of the Prior Art
Prior art methods for picture data reduction can be subdivided into:
1. DPCM (Differential Pulse Code Modulation) methods-transformation methods; and
2. Hybrid methods.
In DPCM methods, the difference between an estimate determined from samples that have already been transmitted and the actual sample is respectively identified. In pure DPCM coders, this prediction occurs three-dimensionally, in other words, both within a frame or picture as well as from frame to frame.
In transformation methods, an imaging of the frame into the transformation region occurs. Due to the high cost, only two dimensional transformations have previously been realized in practice.
The present invention relates to a hybrid method. The principles of a hybrid method is illustrated in FIG. 1. In FIG. 1, a digitized signal x (k, e, t) is supplied to a transformation stage and produces a transformation coefficient signal y(u, v, t) which is supplied to a quantitizer Q which produces a signal Ya(u, v, t) which is supplied through an adder to a coder C which produces a signal Yc(u,v,t) which is supplied as the channel signal. The output of the quantitizer Q is also supplied to a predictor and memory P+M which supplies a signal y.sub.p (u, v,t-1) to an adder to add the signal to the output of the transformation stage before supplying it to the quantitizer Q.
Hybrid coding represents a mixture of transformation and DPCM. The transformation within a frame occurs two-dimensionally, block size 16.times.16 or 8.times.8 picture points, whereas DPCM operates from frame to frame. The signal decorrelated by transformation and/or DPCM is quantitized and transmitted.
Basically, all hybrid methods operate according to the diagram illustrated in FIG. 1. In developed methods, the functions Q, P and C are adaptively executed
European Patent Application No. 82.3070263 discloses a method which employs a coder having the following essential features:
Dynamic bit allocation--The bit rate is minimized and is selected from a plurality of Huffman code tables by means of a prediction algorithm for each coefficient to be coded.
Length of run coding--Zeros successively appearing along a defined scan direction are coded by lengths of run.
Constant Channel rate--Is achieved by coupling the quantitizer to the buffer filling. A PI controller with proportional integrating behavior is employed for this purpose.
The publication of F. May, "Codierung von Bildfolgen mit geringer Rate fur gestorte Uebertrangungskanale", NTG-Fachberichte, Vol. 74, pp. 379-388, describes a system for picture transmission using narrow-band radio channels with a transmission rate of 9.6K bit/s and a frame frequency of 0.5 frames. A plurality of bit allocation matrices are provided for this known method so that the optimum of the respective block is identified and transmitted in the form of a class affiliation. Optimum non-linear quantization characteristics are also employed with respect to the quadratic error. A constant channel rate is achieved by input buffer control, in other words, every frame is first analyzed, the number of coeficients to be transmitted is then modified until the channel rate is observed.
The publication of W. H. Chen, W. K. Pratt entitled "Scene Adaptive Coder", in the IEEE Trans. Comm., Vol. Com32, No. 3, of Mar. 1984, describes an adaptive band width compression technique which employs a discrete cosine transformation. This system is similar to that describes in European Patent Application No. 82.30 70 263 referenced above.
A publication of A. G. Tescher, entitled "Rate Adaptive Communication", appearing in the IEEE International Conference on Communication, of 1978, pages 1.1-19.1.6 describes a concept for a bit rate control in a source coding system.
The technical book publication of W. K. Pratt entitled Image Transformation Techniques, published by the Academic Press, New York, San Francisco, and London in 1979 provides overall discussion of the transformation techniques of the systems.